The Foundations of Chemistry

By convention, oxidation numbersare
represented as
nand n, but ionic
charges are represented as n
and n.
We shall circle oxidation numbers
associated with formulas and show
them in red. Both oxidation numbers
and ionic charges can be combined
algebraically.

Usually the element with the positive
oxidation number is written first.
For historic reasons, however, in
compounds containing nitrogen and
hydrogen, such as NH 3 , and many
compounds containing carbon and
hydrogen, such as CH 4 , hydrogen is
written last, although it has a positive
oxidation number.

140 CHAPTER 4: Some Types of Chemical Reactions

x 2

Solution

(a) The oxidation number of O is 2. The sum of the oxidation numbers for all atoms in a

compound must be zero:

ox. no./atom:

N 2 O 4

total ox. no.: 2x
4(2)0orx 
 4

(b) The oxidation number of H is 
1:

ox. no./atom:

NH 3

total ox. no.: x
3(1)0orx  3

(c) The oxidation number of H is 
1 and the oxidation number of O is 2.

ox. no./atom:

HNO 3

total ox. no.: 1
x
3(2)0orx 
 5

(d) The sum of the oxidation numbers for all atoms in an ion equals the charge on the ion:

ox. no./atom:

NO 3 

total ox. no.: x
3(2)1orx 
 5

(e) The oxidation number of any free element is zero.

You should now work Exercise 38.

CLASSIFYING CHEMICAL REACTIONS

We now discuss chemical reactions in further detail. We classify them as oxidation–

reduction reactions, combination reactions, decomposition reactions, displacement reac-

tions, and metathesis reactions. The last type can be further described as precipitation

reactions, acid–base (neutralization) reactions, and gas-formation reactions. We will see

that many reactions, especially oxidation–reduction reactions, fit into more than one cate-

gory, and that some reactions do not fit neatly into any of them. As we study different

kinds of chemical reactions, we will learn to predict the products of other similar reac-

tions. In Chapter 6 we will describe typical reactions of hydrogen, oxygen, and their

compounds. These reactions will illustrate periodic relationships with respect to chemical

properties. It should be emphasized that our system is not an attempt to transform nature

so that it fits into small categories but rather an effort to give some order to our many

observations of nature.

OXIDATION–REDUCTION REACTIONS: AN

INTRODUCTION

The term “oxidation” originally referred to the combination of a substance with oxygen.

This results in an increase in the oxidation number of an element in that substance.

According to the original definition, the following reactions involve oxidation of the

4-5

x  2

x
 1

 1 x 2